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Strange amplifier problem

Dear mates,
I started to experience a strange problem with my amplifier. Here is the description.

1. I'm listening to the music, everything is OK. Then I make a break for about on hour or two. Want to listen again, but there is no sound, though all tubes are glowing.

2. I check the CD player (change tracks, switch off and on, etc.), it is OK.

3. After some time I see a spark near the IEC inlet and check the fuse - it is blown.

This problem occurs for the second time during the last week. It is obvious that something inside the amp makes the fuse blow.

Some time ago I noticed another problem - when the amp is running, the left output transformer runs warmer than the right one. I wrote the dealer and the manufacturer, but company's president answered that this was normal because the left transformer is closer to the tubes (!!!), though this is not the case - they are symmetrical. The company's president either does not know what his company makes or deliberately ignors my problem in order to rule out any attempts to bring the amp back to him.

My numerous questions about the transformers and fuse remain unanswered.

Before bringing the amp to the dealer, I will appreciate any opinions of those who encountered similar problem and to whom such symptoms are familiar.
transl
transl OP
156 posts
 
This is not a Danish, but an Italian amp.
shadorne's avatar
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shadorne
7,477 posts
 
If the fuse is blowing then you have a power supply problem - a component has failed somewhere or is failing once it gets real heat (things can get very hot on standby on Class A designs - they actually run cooler when operating). When the device fails the power supply draws too much current and blows your fuse. The fact it works again (once you replace the fuse) suggests that the component is not quite dead...yet or that your amp manages to work in a crippled mode until it overheats itself.
shadorne's avatar
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shadorne
7,477 posts
 
Remember the amp cools quickly during the time it takes to replace a fuse and then once you play music again it may run cooler on whichever side of the amp (+ve or -ve signal) that is tending to breakdown.
transl OP
156 posts
 
Yes, exactly. One side still runs cooler.
newbee
4,767 posts
 
Transl, FWIW it is possible that one or more of the tubes in the amp when it was blowing fuses had a 'bias' point which exceded the values of the bias circuit. A personal experience (on two different amps - same tube set). One amp had 'auto bias', one tube ran hot (plate turned red). Sub'ed in another set and this same location (V3) ran hot again. Sent the amp (under warranty) back for servicing. They reset the bias - the amp has worked fine ever since, but with different tubes - I thought the initial tube that tuned red might have been the problem.

Next I tried to use that same set in an amp with fixed bias. I reduced the bias pots to minimum as normal. Three out of the four tubes had a normal (minimum reading), but the tube that had turned red in the other amp registered 50ma on my meter with the pot in minimum position. I subsequently figured out that the correct bias point on this tube should have been 75 and it was somehow mixed into a batch with a 50 bias point.

How, or if that applies to your amp, I don't know. Just don't assume that auto bias circuits and tube bias points assigned by tube matchers are foolproof. I'm very carful now when I buy matched tubes to insist of bias points of about 40 and have had 0 problems since.

Or, it might have been an short caused by an undersized pin, or loose contacts in the socket, causing a short too (although with the latter it was always noise, popping etc, and was corrected by tighting the contacts in the socket.

Lost any fuses since you went back to the original tubes. BTW, what were the tube types and brand of the tubes you were trying to roll in?

FWIW.
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